This invention relates generally to an imaging system, and more specifically to an improved migration imaging member and xeroprinting duplicating process utilizing the improved migration imaging member.
In the art of printing/duplicating, various techniques have been developed for preparing masters for subsequent use in printing processes. For example, lithographic or offset printing is a well known and established printing process. In general, lithography is a method of printing from a printing plate which depends upon different properties of the imaged and non-imaged areas for printability. In conventional lithography, a lithographic intermediate is first prepared on silver halide film from the original; the printing plate is then contact exposed by intense UV light through the intermediate. UV exposure causes the exposed area of the printing plate to become hydrophilic or ink receptive; the non-exposed area is washed away by chemical treatment and becomes hydrophobic or ink repellant. Printing ink is then applied to the printing plate and the ink image is transferred to an offset roller where the actual printing takes place. Although lithographic printing provides high quality prints and high printing speed, the processes require the use of expensive intermediate films and printing plates. Additionally, considerable cost and time are consumed in their preparation, often requiring highly skilled labor and strict control measures. A further disadvantage is the difficulty in setting up the printing press to achieve the proper water to ink balance required to produce the desired results during the printing process. This results in further increased cost and delay time in obtaining the first acceptable print.
The above mentioned problems become especially severe in the manufacture of high quality color prints when several color separation images must be superimposed on the same receiving medium. Because of the high cost and complexities associated with the preparations of expensive printing plates and press runs, color proofing is employed to form representative interim prints (called proofs) from color separation components to allow the end user to determine whether the finished prints faithfully reproduce the desired results. As is often the case, the separation components may require repeated alteration to satisfy the end user. Only when the end user is satisfied with the results, a printing plate associated with each separation component is prepared and ultimately employed in the press run. An example of a color proofing system is the CROMALIN, introduced by E. I. duPont de Nemours & Co. in 1972 and widely used in the printing industry. It consists of a light sensitive tacky photopolymer layer laminated to paper. The photopolymer layer is contact exposed through a color separation component under a UV source. The exposed areas polymerize and lose their tackiness, while the non-exposed areas remain tacky. Toners are applied and adhere to the tacky areas. Since very different processes are employed in proofing and press runs, the proofs at best can only simulate the press sheets. Additionally, preparation of the color proofs is a time consuming process (e.g. about 30 minutes per proof for CROMALIN).
Xerographic printing is another well known printing technique. In conventional xerographic printing, an electrostatic image is first produced, either by lens coupled exposure to visible light or by laser scanning, on a conventional photoreceptor; the electrostatic image is then toned, followed by transferring the toner image to a receiving medium. While it offers the advantages of ease of operation and printing stability, requiring less skilled involvement and labor cost, the combined requirements of high quality and high printing speed, as those needed in commercial printing can not be easily met simultaneously at reasonable cost. This is because, to provide high quality and avoid certain artifacts, very high-picture-element density is also required. If a new image were to be written, for example, on the photoreceptor for each print, these requirements for high speed and high density would imply electronic bandwidths and (if laser scanning were used) modulation rates and polygon rotation speeds which are very unlikely to be available at resonable cost in the foreseeable future. There is no technology likely to overcome this problem in a direct way. The problems relating to conventional xerographic duplicating and printing include the necessity to continually repeat at high speed the imagewise exposure step.
Xeroprinting is another xerographic printing method. Conceptually, xeroprinting overcomes the above problems in a very simple way. Xeroprinting is an electrostatic printing process for printing multiple copies from a master plate or cylinder. The master plate may comprise a metal sheet upon which is imprinted an image in the form of a thin electrically insulating coating. The master plate may be made by photomechanical methods or by xerographic techniques. From the original, a single xeroprinting "master" can, for example, first be made slowly, in say 30-60 seconds. This imaged material is classically an electrical conductor with an imagewise pattern of insulating areas made by photomechanical or xerographic techniques; it has different charge acceptance in the imaged and non-imaged areas. Thus, generally, the imaging surface of the master plate comprises an electrically insulating pattern corresponding to the desired image shape and electrically conductive areas corresponding to the background. The xeroprinting master is then uniformly charged; the carge remains trapped only on the insulating areas, and this electrostatic image may then be toned. After toner transfer to paper and possibly cleaning, the charge-tone-transfer-clean process is repeated at high speed. In principle, then, it is possible to retain much of the simplicity, stability and quality of the xerographic process, without the need for repeated imagewise exposure. As an additional bonus, it may not be necessary to employ a cleaning step, since the same area is repeatedly toned. Moreover, conventional toners can be used, avoiding the problem of lack of color saturation which is encountered with comparable schemes employing magnetography. High contrast potential and high resolution of the electrostatic latent image are important characteristics that determine print qualities of documents prepared by xeroprinting. However these prior art xeroprinting techniques were found to produce prints of inferior quality. This is because an insulating pattern on a metal conductor cannot be fully and uniformly charged near its boundaries. As contrast potential builds up along the boundaries of the insulating pattern, fringing electric fields from the insulating image areas repel incoming ions from the charging device, which is usually a corona charging device, to the adjacent electrically conductive background areas. This results not only in low contrast potential but also in poor print resolution. Additionally, some xeroprinting processes require numerous processing steps and complex equipment to prepare the master and/or final xeroprinted product. Some xeroprinting techniques also require messy photochemical processing and removal of materials in either the image or non-image areas of the master.
In U.S. Pat. No. 3,574,614 issued to L. Carreira, a xeroprinting process is disclosed in which the xeroprinting master is formed by applying an electric field to a layer of photoelectrophoretic imaging suspension between a blocking electrode and an injecting electrode, one of which is transparent, the suspension comprising a plurality of photoelectrophoretic particles in an insulating carrier liquid, imagewise exposing the suspension to electromagnetic radiation through the transparent electrode to form complementary images on the surfaces of the electrodes (the light exposed particles migrating from the injecting electrode to the blocking electrode), transferring one of the images to a conductve substrate, uniformly applying to the image bearing substrate an organic insulating binder such that the binder thickness both within the image formed and the non-image areas ranges from 1-20 micrometers. The xeroprinting process consists of applying a uniform charge to the surface of the image bearing substrate in the presence of electromagnetic radiation to form an electrostatic residual charge pattern corresponding to the non-image areas (areas void of photoelectrophoretic particles), developing the residual charge pattern, transferring the developer from the residual charge pattern to a copy sheet and repeating the charging, developing and transferring steps. Alternatively, the insulating binder may be intimately blended with the dispersion of the photoelectrophoretic particles prior to insertion of the liquid mixture between the electrodes. The areas from which photoelectrophoretic particles have migrated become insulating and capable of supporting an electrostatic charge. A major problem is that insulating images supported directly on a conducting substrate cannot be charged close to the edges, because fringe fields drive incoming ions to the grounded substrate. Another disadvantage of such processes is that they require the use of a liquid photoelectrophoretic imaging suspension to prepare the master. Additionally master making processes are extremely complicated involving the removal of one of the electrodes, transfer of one of the complementary images to a conductive substrate, and application of an organic insulating binder to the conductive substrate. Such complicated master making processes are inconvenient to the users and can adversely affect the print quality. It also requires additional time to dry the image prior to use as a zeroprinting master.
Unlike the liquid photoelectrophoretic imaging suspension system described in U.S. Pat. No. 3,574,614, solid imaging members have been prepared for dry migration systems. Dry migration imaging members have been extensively described in the patent literature, for example, in U.S. Pat. No. 3,909,262 which issued Sept. 30, 1975 and U.S. Pat. No. 3,975,195 which issued Aug. 17, 1976, the disclosures of both being incorporated herein in their entirety. In a typical embodiment of these migration imaging systems, a migration member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material is originally in the form of a fracturable layer contiguous the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer.
The expression "softenable" as used herein in intended to mean any material which can be rendered more permeable thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also near, though not in contact, and adjoining, and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis, the surface of the softenable layer spaced apart from the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (low optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the image on the original.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the image on the original and the light areas of the image formed on the migration imaging member correspond to the dark areas of the image on the original.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this application by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. For the purpose of this invention, all value of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate.
There are various other systems for forming such images, where non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patent, which also discloses a variety of methods which may be used to form latent images upon migration imaging members.
Various means for developing the latent images may be used for migration imaging systems. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates alone, this region exhibits a maximum optical density which can be high as the initial optical density of the unprocessed film. On the other hand, the migration marking material inthe unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore the image sense of the developed image is sign reversed, i.e. positive to negative or vice versa. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat, or vapor softening developing modes, the migration marking material in the light struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6-0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density (D.sub.max) of about 1.8-1.9. Therefore, the image sense of the heat or vapor developed images is sign retaining, i.e. positive-to-positive or negative-to-negative.
Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be found in U.S. Pat. No. 3,795,512.
For many imaging applications, it is desirable to produce negative images from a positive original or positive images from a negative original i.e. optically sign-reversing imaging, preferably with low minimum optical density. Although the meniscus or solvent wash away development method produces optically sign-reversed images with low minimum optical density, it involves removal of materials from the migration imaging member, leaving the migration image largely or totally unprotected from abrasion. Although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step is impractically costly and inconvenient for the end users. Additionally, disposal of the effluents washed from the migration imaging member during development is also very costly.
The background portions of an imaged member may sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and the softenable layer softened by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85-90% of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25-0.35 range. Thus the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image may be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the agglomerate towards a shape of lower energy, such as a sphere.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the film and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in U.S. Pat. No. 3,909,262. However, because the migration imaging mechanisms for each development method are different and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles etc., application of an overcoat to the softenable layer often causes changes in the delicate balance of these processes, and results in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density is degraded. Recently, improvements in migration imaging members and processes for forming images on these migration imaging members have been achieved. These improved migration imaging members and processes are described in U.S. Pat. No. 4,536,458 issued to Dominic S. Ng and U.S. Pat. No. 4,536,457 issued to Man C. Tam.